CN113574215B - Base substrate and method for manufacturing the same - Google Patents

Abstract

The present invention provides a high-quality base substrate provided with an alignment layer for crystal growth of a nitride or oxide of a group 13 element, wherein crystal defects (dislocations) in the alignment layer are significantly reduced. The base substrate is provided with an orientation layer for crystal growth of a nitride or oxide of a group 13 element. The surface of the orientation layer on the side for crystal growth is composed of a material having a corundum crystal structure with a-axis length and/or c-axis length greater than that of sapphire. The orientation layer comprises: contains 2 or more solid solutions selected from the group consisting of α－Al₂O₃、α－Cr₂O₃、α－Fe₂O₃、α－Ti₂O₃、α－V₂O₃ and a-Rh ₂O₃.

Description

Base substrate and method for manufacturing the same

Technical Field

The present invention relates to a base substrate for growing a nitride or oxide crystal of a Group 13 element and a method for manufacturing the same.

Background technique

In recent years, semiconductor devices using gallium nitride (GaN) have been put into practical use. For example, devices formed by alternately stacking n-type GaN layers, quantum well layers containing InGaN layers, and barrier layers containing GaN layers in sequence on a sapphire substrate to form a multi-quantum well layer (MQW) and a p-type GaN layer have been mass-produced.

In addition, research and development of α-gallium oxide (α-Ga ₂ O ₃ ) of the corundum phase type having the same crystal structure as sapphire is also being actively carried out. In fact, the band gap of α-Ga ₂ O ₃ is as high as 5.3 eV, and it is highly anticipated as a material for power semiconductor elements. For example, Patent Document 1 (Japanese Patent Publication No. 2014-72533) relates to a semiconductor device, which is formed by a base substrate having a corundum-type crystal structure, a semiconductor layer having a corundum-type crystal structure, and an insulating film having a corundum-type crystal structure, and discloses an example of forming a film of α-Ga ₂ O ₃ as a semiconductor layer on a sapphire substrate. In addition, Patent Document 2 (Japanese Patent Publication No. 2016-25256) discloses the following content, namely, a semiconductor device comprising: an n-type semiconductor layer including a crystalline oxide semiconductor having a corundum structure as a main component, a p-type semiconductor layer including an inorganic compound having a hexagonal crystal structure as a main _component , and an electrode. In an embodiment, a metastable phase, i.e., α- _Ga2O3 having a corundum structure is formed on a c-plane sapphire substrate as the n-type semiconductor layer and an α- _{Rh2O3 film} having a hexagonal crystal structure is formed as the p-type semiconductor layer to produce a diode.

However, it is known that in these semiconductor devices, materials with fewer crystal defects can obtain good characteristics. In particular, power semiconductors are required to have excellent voltage resistance characteristics, so it is desirable to reduce crystal defects. This is because the number of crystal defects will affect the dielectric breakdown electric field characteristics. However, for GaN and α-Ga ₂ O ₃ , single crystal substrates with fewer crystal defects have not yet been put to practical use, and are usually formed by heteroepitaxial growth on sapphire substrates with a lattice constant different from these materials. Therefore, crystal defects are easily generated due to the difference in lattice constants with sapphire. For example, when α-Ga ₂ O ₃ is formed into a film on the c-plane of sapphire, the a-axis length of sapphire (α-Al ₂ O ₃ ) and the a-axis length of α-Ga ₂ O ₃ The difference is about 4.8%, which constitutes the main reason for crystal defects.

As a method of alleviating the above-mentioned lattice constant difference with the semiconductor layer to reduce crystal defects, it has been reported that when forming an α-Ga ₂ O ₃ film, a buffer layer is formed between the sapphire and α-Ga ₂ O ₃ layers, thereby reducing defects. For example, Non-Patent Document 1 (Applied Physics Express, vol. 9, pages 071101-1 to 071101-4) gives an example in which an (Al _x , Ga _1-x ) ₂ O ₃ layer (x=0.2 to 0.9) is introduced as a buffer layer between the sapphire and α-Ga ₂ O ₃ layers, so that the edge dislocations and screw dislocations are 3×10 ⁸ /cm ² and 6×10 ⁸ /cm ² , respectively. In addition, Non-Patent Document 2 (Applied Physics Express 11, 111101 (2018)) discloses a substrate having an α-Cr ₂ O ₃ film formed on sapphire as a buffer layer as a substrate for forming an α-Ga ₂ O ₃ film.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2014-72533

Patent Document 2: Japanese Patent Application Publication No. 2016-25256

Non-patent literature

Non-patent document 1: Riena Jinno et al., Reduction in edge dislocation density in corundum-structuredα-Ga2O3 layers on sapphire substrates with quasi-gradedα-(Al,Ga)2O3 buffer layers, Applied Physics Express, Japan, The Japan Society of Applied Physics ,June 1,2016,vol.9,pages 071101-1to 071101-4

Non-patent document 2: Giang T.Dang et al., Growth of α-Cr2O3 single crystals bymist CVD using ammonium dichromate, Applied Physics Express 11, 111101 (2018)

Summary of the invention

However, the method of introducing a buffer layer disclosed in non-patent document 1 and non-patent document 2 is not sufficient for applications in power semiconductors that require high dielectric breakdown electric field characteristics, and it is desired to further reduce crystal defects. In the case of forming α-Cr ₂ O ₃ as a buffer layer disclosed in non-patent document 2, it is also insufficient for applications in power semiconductors that require high dielectric breakdown electric field characteristics, and it is desired to further reduce crystal defects. When an α-Cr ₂ O ₃ film is used as a buffer layer, the reasons why crystal defects cannot be sufficiently reduced are speculated to be: i) there is a lattice mismatch between α-Cr ₂ O ₃ and α-Ga ₂ O ₃ , and ii) a relatively thin buffer layer is formed on sapphire by heteroepitaxial growth. Due to such a structure, the buffer layer contains relatively large crystal defects.

The inventors of the present invention have recently obtained the finding that an excellent semiconductor layer can be formed by using a base substrate having an orientation layer whose surface on the side for growing nitride or oxide crystals of a Group 13 element is made of a material having a corundum-type crystal structure whose a-axis length and/or c-axis length is larger than that of sapphire, and the orientation layer contains: a solid solution containing two or more selected from the group consisting of α-Al ₂ O ₃ , α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ and α-Rh ₂ O ₃ .

Therefore, an object of the present invention is to provide a high-quality base substrate having an orientation layer for growing nitride or oxide crystals of a Group 13 element and having significantly reduced crystal defects (dislocations) in the orientation layer. In addition, another object of the present invention is to provide a method for manufacturing such a base substrate.

According to one aspect of the present invention, there is provided a base substrate having an orientation layer for growing a nitride or oxide crystal of a Group 13 element.

The base substrate is characterized in that

The surface of the orientation layer on the side for crystal growth is composed of a material having a corundum type crystal structure having an a-axis length and/or a c-axis length greater than that of sapphire,

The alignment layer includes a solid solution containing two or more selected from the group consisting of α-Al ₂ O ₃ , α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ , and α-Rh ₂ O ₃ .

According to another embodiment of the present invention, there is provided a method for manufacturing the base substrate, characterized in that it comprises the following steps:

Preparing a sapphire substrate;

forming an oriented precursor layer comprising the following material on the surface of the sapphire substrate, the material being a material having a corundum type crystal structure whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire, or a material having a corundum type crystal structure whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire through heat treatment; and

The sapphire substrate and the alignment precursor layer are heat-treated at a temperature above 1000°C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the structure of an aerosol deposition (AD) device.

FIG. 2 is a schematic cross-sectional view showing the structure of an atomizing CVD device.

FIG. 3 is a diagram schematically showing the steps of manufacturing the composite base substrate in Example 1. FIG.

FIG. 4 is a diagram schematically showing a process for preparing a sample for evaluating the back surface of an alignment layer in Example 1. FIG.

Detailed ways

Base substrate

The base substrate of the present invention is: a base substrate having an orientation layer for crystal growth of nitrides or oxides of group 13 elements. That is, the base substrate is used to crystal grow a semiconductor layer composed of nitrides or oxides of group 13 elements on the orientation layer. Here, group 13 elements are group 13 elements in the periodic table established by IUPAC (International Union of Pure and Applied Chemistry), specifically any one of boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl) and niobium (Nh). In addition, typical examples of nitrides and oxides of group 13 elements are gallium nitride (GaN) and α-gallium oxide (α-Ga ₂ O ₃ ).

The orientation layer has a structure in which the crystal orientation is substantially consistent in the approximate normal direction. By adopting such a structure, a semiconductor layer with excellent quality, especially excellent orientation, can be formed thereon. That is, when a semiconductor layer is formed on the orientation layer, the crystal orientation of the semiconductor layer substantially follows the crystal orientation of the orientation layer. Therefore, by making the base substrate have a structure with an orientation layer, the semiconductor film can be an orientation film. It should be noted that the orientation layer can be polycrystalline, mosaic crystal (a collection of crystals with a crystal orientation deviating from a number), or a single crystal. In the case where the orientation layer is polycrystalline, it is preferably a biaxial orientation layer in which the twisting direction (i.e., the rotation direction centered on the substrate normal line which is oriented substantially perpendicular to the substrate surface) is also substantially consistent.

The surface of the orientation layer on the side for crystal growth (hereinafter, sometimes simply referred to as "surface" or "orientation layer surface") is composed of a material having a corundum-type crystal structure having an a-axis length and/or a c-axis length greater than that of sapphire (α-Al ₂ O ₃ ). By controlling the lattice constant of the orientation layer in this way, the crystal defects in the semiconductor layer formed thereon can be significantly reduced. That is, the lattice constant of the oxide of the Group 13 element constituting the semiconductor layer is greater than that of sapphire (α-Al ₂ O ₃ ). In fact, as shown in Table 1 below, the lattice constant (a-axis length and c-axis length) of α-Ga ₂ O ₃ , which is an oxide of the Group 13 element, is greater than that of α-Al ₂ O _3. Therefore, by controlling the lattice constant of the orientation layer to be greater than that of α-Al ₂ O ₃ , when a semiconductor layer is formed on the orientation layer, the mismatch of the lattice constant between the semiconductor layer and the orientation layer is suppressed, resulting in a reduction in the crystal defects in the semiconductor layer. For example, when α-Ga ₂ O ₃ is formed into a film on the c-plane of sapphire, the lattice length (a-axis length) in the in-plane direction of α-Ga ₂ O ₃ is larger than the lattice length of sapphire, with a mismatch of about 4.8%. Therefore, by controlling the a-axis length of the orientation layer to be larger than the a-axis length of α-Al ₂ O ₃ , the crystal defects in the α-Ga ₂ O ₃ layer are reduced. When α-Ga ₂ O ₃ is formed into a film on the m-plane of sapphire, the lattice length (c-axis length and a-axis length) in the in-plane direction of α-Ga ₂ O ₃ is larger than the lattice length of sapphire, the c-axis length has a mismatch of about 3.4%, and the a-axis length has a mismatch of about 4.8%. Therefore, by controlling the c-axis length and a-axis length of the orientation layer to be larger than the c-axis length and a-axis length of α-Al ₂ O ₃ , the crystal defects in the α-Ga ₂ O ₃ layer are reduced. In addition, the lattice constant mismatch between GaN, which is a nitride of a group 13 element, and sapphire is also large. When GaN is formed into a film on the c-plane of sapphire, the lattice length (a-axis length) of GaN in the in-plane direction is substantially larger than the lattice length of sapphire, with a mismatch of about 16.2%. Therefore, by controlling the a-axis length of the orientation layer to be larger than the a-axis length of α-Al ₂ O ₃ , the crystal defects in the GaN layer are reduced. In contrast, if these semiconductor layers are formed directly on a sapphire substrate, stress is generated in the semiconductor layer due to the mismatch of the lattice constant, and a large number of crystal defects may be generated in the semiconductor layer.

[Table 1]

Table 1: Lattice constants of group 13 oxides

It is preferred that the entire orientation layer is composed of a material having a corundum type crystal structure. Accordingly, the crystal defects of the orientation layer and the semiconductor layer can be reduced. The orientation layer is preferably formed on the surface of a sapphire substrate. The α-Al ₂ O ₃ constituting the sapphire substrate has a corundum type crystal structure. By forming the orientation layer with a material having a corundum type crystal structure, it can be made to have the same crystal structure as the sapphire substrate. As a result, the crystal defects caused by the mismatch of the crystal structure in the orientation layer are suppressed. In this regard, if the crystal defects in the orientation layer are reduced, the crystal defects in the semiconductor layer formed thereon are also reduced, so it is preferred. This is because: if there are a large number of crystal defects in the orientation layer, the crystal defects are also inherited in the semiconductor layer formed thereon, and as a result, crystal defects are also generated in the semiconductor layer.

The material having a corundum type crystal structure constituting the orientation layer preferably includes: a solid solution containing two or more selected from the group consisting of α-Al ₂ O ₃ , α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ and α-Rh ₂ O _3. As shown in Table 1 above, these materials have a lattice constant (a-axis length and/or c-axis length) larger than that of α-Al ₂ O ₃ , and the lattice constant is relatively close to or consistent with the lattice constant of the nitride or oxide of the 13th group element constituting the semiconductor layer, that is, GaN or α-Ga ₂ O ₃ , so that the crystal defects in the semiconductor layer can be effectively suppressed. As the above-mentioned solid solution, any of a substitutional solid solution and an intrusive solid solution may be used, but a substitutional solid solution is preferred. However, although the orientation layer is composed of a material having a corundum type crystal structure, it is not excluded that it contains trace components other than these.

A particularly preferred orientation layer is composed of a material containing a solid solution of α-Cr ₂ O ₃ and a different material. In this case, the base substrate (particularly the orientation layer) can be well used for the crystal growth of a semiconductor layer composed of α-Ga ₂ O ₃ or an α-Ga ₂ O ₃ -based solid solution. That is, in the crystal growth of α-Ga ₂ O ₃ or an α-Ga ₂ O ₃ -based solid solution, if a base substrate having an orientation layer composed of a solid solution of α-Cr ₂ O ₃ and a different material on at least one surface on which the film is formed is used is used, an excellent semiconductor layer composed of α-Ga ₂ O ₃ or an α-Ga ₂ O ₃ -based solid solution can be formed thereon.

The a-axis length of the material having a corundum type crystal structure on the surface of the orientation layer for crystal growth is greater than And for The following are more preferably More preferably In addition, the c-axis length of the material having a corundum type crystal structure on the surface of the orientation layer on the side for crystal growth is greater than And for The following are more preferably More preferably By controlling the a-axis length and/or c-axis length of the orientation layer surface within the above range, it can be made close to the lattice constant (a-axis length and/or c-axis length) of the nitride or oxide of the Group 13 element constituting the semiconductor layer, particularly α-Ga ₂ O ₃ .

The thickness of the orientation layer is preferably 10 μm or more, more preferably 40 μm or more. The upper limit of the thickness is not particularly limited, and is typically 1000 μm or less. When the orientation layer is used alone as a self-supporting substrate, from the perspective of operability, the orientation layer can be thicker, for example, the thickness can be 1 mm or more, but from the perspective of cost, the thickness is, for example, 2 mm or less. By making the orientation layer thicker in this way, the crystal defects on the surface of the orientation layer can also be reduced. In the case of forming an orientation layer on a sapphire substrate, the lattice constants of the sapphire substrate and the orientation layer differ slightly, and as a result, crystal defects are easily generated at their interface, that is, at the bottom of the orientation layer. However, by making the orientation layer thicker, the influence of crystal defects generated in the lower part of the orientation layer can be reduced on the surface of the orientation layer. The reason is uncertain, but it is believed that it is because: the crystal defects generated in the lower part of the orientation layer disappear before reaching the surface of the thicker orientation layer. In addition, by making the orientation layer thicker, the following effect can also be expected, that is, after forming a semiconductor layer on the orientation layer, the semiconductor layer is peeled off and the base substrate can be reused. The crystal defect density in the surface of the orientation layer is preferably 1.0×10 ⁸ /cm ² or less, more preferably 1.0×10 ⁶ /cm ² or less, and further preferably 4.0×10 ³ /cm ² or less, and there is no particular lower limit. It should be noted that in this specification, crystal defects refer to: threading edge dislocations, threading screw dislocations, threading mixed dislocations, and basal plane dislocations, and the crystal defect density is the sum of the densities of each dislocation. In addition, basal plane dislocations constitute a problem when the base substrate including the orientation layer has an off-angle, and do not constitute a problem when there is no off-angle because they are not exposed to the surface of the orientation layer. For example, if the material contains 3×10 ⁸ /cm ² threading edge dislocations, 6×10 ⁸ /cm ² threading screw dislocations, and 4×10 ⁸ /cm ² threading mixed dislocations, the crystal defect density is 1.3×10 ⁹ /cm ² .

When the orientation layer is formed on a sapphire substrate, it is preferred that a gradient composition region in which the composition changes in the thickness direction exists in the orientation layer. For example, the gradient composition region preferably includes a region (gradient composition region) composed of a solid solution of α-Al 2 O 3 and one or more materials selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ and α-Rh ₂ O ₃ , and having a gradient composition in which the solid solution amount of α-Al _{2 O 3 decreases} from the sapphire substrate side toward the _orientation layer surface side. The gradient composition region is preferably composed of a solid solution containing α-Al ₂ O ₃ and α-Cr ₂ O ₃ , and is particularly preferably composed of a solid solution containing α-Al ₂ O ₃ , α-Cr ₂ O ₃ and α-Ti ₂ O ₃ or a solid solution containing α-Al ₂ O ₃ , α-Cr ₂ O ₃ , α-Ti ₂ O ₃ and α-Fe ₂ O _3. That is, the orientation layer is preferably formed on the surface of the sapphire substrate, which has the effect of alleviating the stress caused by the difference in lattice constant (a-axis length and/or c-axis length) between the sapphire substrate and the orientation layer and suppressing crystal defects. In other words, it is preferred that the a-axis length and/or c-axis length are different on the surface and back of the orientation layer, and it is more preferred that the a-axis length and/or c-axis length on the surface of the orientation layer is greater than the a-axis length and/or c-axis length on the back of the orientation layer. By adopting such a structure, whether the orientation layer is a single crystal or a mosaic crystal, or a biaxial orientation layer, the lattice constant changes in the thickness direction. The difference in a-axis length and/or c-axis length between the surface and the back of the orientation layer is preferably 2.5% or more, more preferably 4.0% or more (the upper limit of this value is not particularly limited, and is typically 15.4% or less). Therefore, a single crystal or mosaic crystal or a biaxial orientation layer can be formed on a substrate with different lattice constants in a state where stress is relaxed. In the manufacture of the base substrate described later, a gradient composition region like this can be formed by heat treating the sapphire substrate and the orientation precursor layer at a temperature above 1000°C. That is, if heat treatment is performed at such a high temperature, a reaction occurs at the interface between the sapphire substrate and the orientation precursor layer, so that the Al component in the sapphire substrate diffuses into the orientation precursor layer, or the component in the orientation precursor layer diffuses into the sapphire substrate. As a result, a gradient composition region is formed in which the solid solution amount of α-Al ₂ O ₃ changes in the thickness direction. For the gradient composition region, the thicker one is easier to relax the stress caused by the difference in lattice constants, so it is preferably thicker. Therefore, the thickness of the gradient composition region is preferably 5μm or more, and more preferably 20μm or more. The upper limit of the thickness is not particularly limited, and is typically less than 100 μm. In addition, by performing a heat treatment at 1000°C or above, the crystal defects reaching the surface of the orientation layer can be effectively reduced. The reason for this is uncertain, but it is believed that the high temperature heat treatment promotes the cancellation of crystal defects.

According to a more preferred embodiment of the present invention, the orientation layer has: a compositionally stable region, which is located near the surface and has a stable composition in the thickness direction; and a gradient composition region, which is located away from the surface and has a composition that changes in the thickness direction. The compositionally stable region refers to a region where the change in the content ratio of each metal element is less than 1.0at%, and the gradient composition region refers to a region where the change in the content ratio of each metal element is greater than 1.0at%. For example, the compositionally stable region and the gradient composition region can be determined as follows. First, prepare a cross-sectional sample of the orientation layer, perform energy dispersive X-ray analysis (EDS) at any 10 locations near the surface of the orientation layer, and calculate the average content ratio (at%) of the detected metal elements. Next, perform EDS analysis at any 10 locations 2μm away from the surface in the thickness direction, and calculate the average content ratio (at%) at a thickness of 2μm. At this time, the average values of the content ratios at the surface and at a thickness of 2 μm are compared. Depending on whether the difference in the content ratio of at least one of all the detected metal elements is less than 1.0 at% or greater than 1.0 at%, the area from the surface to a thickness of 2 μm can be assigned to either a compositionally stable area or a gradient composition area. Using the same method, the average value of the content ratio of the metal element is calculated every 2 μm in the thickness direction, and the average value of the content ratio of the metal element between a certain thickness position and a position 2 μm away from the thickness position in the thickness direction is compared, thereby determining the attribution of the area between the positions. For example, for the area between a position with a thickness of 24 μm from the surface and a position with a thickness of 26 μm, the attribution can be determined by calculating the average value of the content ratio of the metal element at each position and comparing them. Then, for example, in the case where the orientation layer contains Al, it is more preferred that the Al concentration in the gradient composition area tends to decrease in the thickness direction toward the compositionally stable area. In this embodiment, the material having a corundum type crystal structure is preferably a solid solution containing two or more materials selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , and α-Ti ₂ O ₃ , or a solid solution containing α-Al ₂ O ₃ and one or more materials selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , and α-Ti ₂ O _3. In particular, it is preferred that the gradient composition region is composed of a solid solution containing α-Cr ₂ O ₃ and α-Al ₂ O _3. In addition, the composition stable region may be a material having a lattice constant (a-axis length and/or c-axis length) larger than the lattice constant of α-Al ₂ O ₃ , and may be a solid solution between a plurality of corundum type materials, or may be a single phase of a corundum type material. That is, the material constituting the compositionally stable region is preferably: (i) a solid solution containing two or more materials selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ and α-Rh ₂ O ₃ , or (ii) a solid solution of one or more materials selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ and α-Rh ₂ O ₃ and α-Al ₂ O _3. From the viewpoint of controlling the lattice constant, it is preferably composed of a solid solution of a material that does not contain α-Al ₂ O ₃ among the above materials.

The material constituting the orientation layer is not particularly limited as long as it has orientation relative to the surface of the base substrate, for example, c-axis orientation, a-axis orientation, or m-axis orientation. Accordingly, when a semiconductor layer is formed on the base substrate, the semiconductor film can be a c-axis orientation film, a-axis orientation, or m-axis orientation.

The orientation layer is preferably a heteroepitaxial growth layer. For example, when the orientation layer is grown on a sapphire substrate, both the sapphire substrate and the orientation layer have a corundum-type crystal structure. Therefore, when their lattice constants are close, epitaxial growth occurs during heat treatment in which the crystal plane of the orientation layer is arranged in accordance with the crystal orientation of the sapphire substrate. By epitaxially growing the orientation layer in this way, the orientation layer can inherit the high crystallinity and crystal orientation unique to the single crystal of the sapphire substrate.

The arithmetic mean roughness Ra of the surface of the alignment layer is preferably 1 nm or less, more preferably 0.5 nm or less, and further preferably 0.2 nm or less. It is considered that smoothing the surface of the alignment layer in this way further improves the crystallinity of the semiconductor layer provided thereon.

The single side of the base substrate preferably has an area of ²⁰ cm2 or more, more preferably 70 ^cm2 or more, and further preferably 170 ^cm2 or more. By making the base substrate larger in area, the semiconductor layer formed thereon can be larger in area. Therefore, a plurality of semiconductor elements can be obtained from a single semiconductor layer, and further reduction in manufacturing cost can be expected. The upper limit of the size is not particularly limited, and is typically ⁷⁰⁰ cm2 or less on a single side.

The base substrate of the present invention preferably further comprises a supporting substrate on the side of the orientation layer opposite to the surface (i.e., the back side). That is, the base substrate of the present invention may be: a base substrate comprising a supporting substrate and an orientation layer disposed on the supporting substrate. Furthermore, the supporting substrate is preferably a sapphire substrate, a corundum single crystal such as Cr ₂ O ₃ , and is particularly preferably a sapphire substrate. By making the supporting substrate a corundum single crystal, it can also serve as a seed for heteroepitaxial growth of the orientation layer. In addition, by adopting a structure comprising a corundum single crystal in this way, a semiconductor layer of excellent quality can be obtained. That is, a corundum single crystal has excellent mechanical properties, thermal properties, chemical stability and other characteristics. In particular, the thermal conductivity of sapphire is as high as 42 W/m·K at room temperature, and the thermal conductivity is excellent. Therefore, by adopting a base substrate comprising a sapphire substrate, the thermal conductivity of the entire substrate can be made excellent. As a result, it can be expected that when a semiconductor layer is formed on the base substrate, the uneven temperature distribution within the substrate surface is suppressed, and a semiconductor layer with a uniform film thickness can be obtained. In addition, there is an effect that a sapphire substrate can be easily obtained in a large area, the overall cost can be reduced, and a semiconductor layer with a large area can be obtained.

The sapphire substrate used as a supporting substrate may have any azimuth plane. That is, it may have an a-plane, a c-plane, an r-plane, or an m-plane, and may have a predetermined angle relative to these planes. In addition, in order to adjust the electrical characteristics, the sapphire substrate may be doped with a dopant. As the dopant, a known dopant may be used.

By using the orientation layer of the base substrate of the present invention, a semiconductor layer composed of a nitride or oxide of a group 13 element can be formed. The method for forming the semiconductor layer can be a known method, but preferably any one of various vapor phase film forming methods such as CVD method, HVPE method, sublimation method, MBE method, PLD method and sputtering method, hydrothermal method, Na flux method and other liquid phase film forming methods. Examples of CVD methods include: thermal CVD method, plasma CVD method, atomized CVD method, MO (organic metal) CVD method, etc. Among them, in order to form a semiconductor layer composed of an oxide of a group 13 element, atomized CVD method, hydrothermal method, or HVPE method is particularly preferred.

The base substrate of the present invention can be in the form of a self-supporting substrate with an orientation layer alone, or in the form of a base substrate accompanied by a supporting substrate such as a sapphire substrate. Therefore, the orientation layer can be finally separated from a supporting substrate such as a sapphire substrate as needed. The separation of the supporting substrate can be carried out by a known method and is not particularly limited. For example, there can be cited: a method of separating the orientation layer by applying a mechanical impact, a method of separating the orientation layer by applying heat and utilizing thermal stress, a method of separating the orientation layer by applying vibrations such as ultrasound, a method of separating the orientation layer by etching the unnecessary part, a method of separating the orientation layer by laser stripping, a method of separating the orientation layer by mechanical processing such as cutting and grinding, etc. In addition, in the case of heteroepitaxial growth of the orientation layer on a sapphire substrate, the sapphire substrate can be separated and the orientation layer can be set on other supporting substrates. The material of other supporting substrates is not particularly limited, and from the perspective of material properties, a suitable material can be selected. For example, from the perspective of thermal conductivity, there can be cited: metal substrates or substrates such as Cu, ceramic substrates such as SiC, AlN, etc.

Manufacturing method

The base substrate of the present invention can be preferably manufactured as follows, that is, (a) preparing a sapphire substrate; (b) making a predetermined orientation precursor layer; (c) on the sapphire substrate, heat-treating the orientation precursor layer to convert at least a portion of the orientation precursor layer near the sapphire substrate into an orientation layer; (d) applying grinding, polishing and other processing as desired to expose the surface of the orientation layer. The orientation precursor layer becomes an orientation layer by heat treatment, which contains: a material having a corundum-type crystal structure with an a-axis length and/or a c-axis length greater than the a-axis length and/or c-axis length of sapphire, or a material having a corundum-type crystal structure with an a-axis length and/or a c-axis length greater than the a-axis length and/or c-axis length of sapphire by the heat treatment described later. In addition, the orientation precursor layer may also contain trace components in addition to the material having the corundum-type crystal structure. According to this manufacturing method, the growth of the orientation layer can be promoted by using the sapphire substrate as a seed. That is, the high crystallinity and crystal orientation orientation peculiar to the single crystal of the sapphire substrate are inherited by the orientation layer.

(a) Preparation of sapphire substrate

In order to make a base substrate, first, a sapphire substrate is prepared. The sapphire substrate used can have any azimuth plane. That is, it can have an a-plane, a c-plane, an r-plane, and an m-plane, and can also have a specified angle relative to these planes. For example, when using c-plane sapphire, the c-axis is oriented relative to the surface, so it is easy to heteroepitaxially grow an oriented layer with a c-axis orientation thereon. In addition, in order to adjust the electrical properties, a sapphire substrate to which a dopant is added can also be used. As the dopant, a known dopant can be used.

(b) Preparation of oriented precursor layer

An oriented precursor layer is prepared containing a material having a corundum type crystal structure whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire, or a material whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire by heat treatment. The method for forming the oriented precursor layer is not particularly limited, and a known method can be adopted. Examples of methods for forming the oriented precursor layer include: AD (aerosol deposition) method, sol-gel method, hydrothermal method, sputtering method, evaporation method, various CVD (chemical vapor growth) methods, HVPE method, PLD method, CVT (chemical vapor transport) method, sublimation method, etc. Examples of CVD methods include: thermal CVD method, plasma CVD method, atomization CVD method, MO (organic metal) CVD method, etc. Alternatively, it can be the following method, that is, a molded body of the oriented precursor is prepared in advance, and the molded body is placed on a sapphire substrate. The oriented precursor material can be formed by tape casting or press molding to produce the molded body. In addition, the following method can be adopted, that is, as the oriented precursor layer, a polycrystalline material previously produced by various CVD methods, sintering, etc. is used and placed on a sapphire substrate.

However, aerosol deposition (AD) method, various CVD methods or sputtering methods are preferred. By adopting these methods, a dense oriented precursor layer can be formed in a relatively short time, and it is easy to perform heteroepitaxial growth with a sapphire substrate as a seed. In particular, the AD method does not require a high vacuum process, and the film forming speed is relatively fast, so it is also ideal in terms of manufacturing cost. In the case of using the sputtering method, a target of the same material as the oriented precursor layer can also be used for film formation, but a reactive sputtering method using a metal target to form a film under an oxygen atmosphere can also be used. The method of placing a pre-made molded body on sapphire is also preferred as a simple method, but since the oriented precursor layer is not dense, a densification process is required in the heat treatment process described later. In the method of using a pre-made polycrystalline as an oriented precursor layer, two processes are required: a process of making polycrystalline and a process of heat treating on a sapphire substrate. In addition, in order to improve the adhesion of the polycrystalline to the sapphire substrate, it is also necessary to make the surface of the polycrystalline fully smooth in advance. Any of the methods can adopt known conditions, but the following describes a method of directly forming an alignment precursor layer by the AD method and a method of placing a previously produced molded body on a sapphire substrate.

The AD method is a technique in which particles or particle raw materials are mixed with a gas to form an aerosol, and the aerosol is ejected at high speed from a nozzle to collide with a substrate to form a film. The AD method has the characteristic of being able to form a dense film at room temperature. An example of a film forming device (aerosol deposition (AD) device) used in the AD method is shown in FIG1 . The film forming device 20 shown in FIG1 is configured as a device used in the AD method in which a raw material powder is ejected onto a substrate in an atmosphere where the pressure is lower than the atmospheric pressure. The film forming device 20 comprises: an aerosol generating unit 22 that generates an aerosol of a raw material powder containing a raw material component; and a film forming unit 30 that ejects the raw material powder onto a sapphire substrate 21 to form a film containing the raw material component. The aerosol generating section 22 includes: an aerosol generating chamber 23 that stores raw material powder and generates aerosol by receiving carrier gas supplied from a gas cylinder (not shown); a raw material supply pipe 24 that supplies the generated aerosol to the film forming section 30; and an exciter 25 that applies vibration to the aerosol generating chamber 23 and the aerosol therein at a vibration frequency of 10 to 100 Hz. The film forming section 30 includes: a film forming chamber 32 that sprays aerosol toward the sapphire substrate 21; a substrate holder 34 that is disposed inside the film forming chamber 32 and fixes the sapphire substrate 21; and an X-Y stage 33 that moves the substrate holder 34 in the X-axis-Y-axis direction. In addition, the film forming section 30 includes: an injection nozzle 36 that has a slit 37 formed at the front end and sprays aerosol toward the sapphire substrate 21; and a vacuum pump 38 that decompresses the film forming chamber 32.

It is known that the film thickness, film quality, etc. can be controlled by the film forming conditions for the AD method. For example, the morphology of the AD film is easily affected by the collision speed of the raw material powder against the substrate, the particle size of the raw material powder, the aggregation state of the raw material powder in the aerosol, the injection amount per unit time, etc. The collision speed of the raw material powder against the substrate is affected by the differential pressure in the film forming chamber 32 and the injection nozzle 36, or the opening area of the injection nozzle, etc. If appropriate conditions are not adopted, the film may become a powder compact or generate pores, so it is necessary to appropriately control these factors.

In the case of using a molded body in which an oriented precursor layer is prepared in advance, the raw material powder of the oriented precursor can be molded to prepare a molded body. For example, in the case of press molding, the oriented precursor layer is a press molded body. The raw material powder of the oriented precursor can be press molded to prepare a press molded body based on a known method. For example, the raw material powder is placed in a mold and pressed at a pressure of preferably 100 to 400 kgf/ ^cm2 , more preferably 150 to 300 kgf/ ^cm2 . In addition, the molding method is not particularly limited. In addition to press molding, tape casting, casting, extrusion molding, scraper method and any combination of these methods can also be used. For example, in the case of tape casting, it is preferred to appropriately add additives such as a binder, a plasticizer, a dispersant, and a dispersion medium to the raw material powder to slurry, so that the slurry passes through a slit-shaped finer ejection port, thereby ejecting and molding in a sheet shape. The thickness of the sheet-shaped molded body is not limited, but from the perspective of operation, it is preferably 5 to 500 μm. When a thicker oriented precursor layer is required, a plurality of the sheet molded products may be stacked and used in a desired thickness.

For these molded bodies, the portion near the sapphire substrate is made into an orientation layer by subsequent heat treatment on the sapphire substrate. As described above, in this method, it is necessary to sinter and densify the molded body in the heat treatment step described later. Therefore, in addition to the material having or bringing the corundum type crystal structure, the molded body may also contain trace components such as a sintering aid.

(c) Heat treatment of the oriented precursor layer on the sapphire substrate

The sapphire substrate formed with the orientation precursor layer is heat-treated at a temperature above 1000°C. Through this heat treatment, at least the portion of the orientation precursor layer near the sapphire substrate can be converted into a dense orientation layer. In addition, through this heat treatment, the orientation layer can be heteroepitaxially grown. That is, by forming the orientation layer with a material having a corundum-type crystal structure, heteroepitaxial growth occurs during heat treatment, in which the material having a corundum-type crystal structure grows crystals with the sapphire substrate as a seed. At this time, rearrangement of the crystals occurs, and the crystals are arranged in accordance with the crystal plane of the sapphire substrate. As a result, the crystal axes of the sapphire substrate and the orientation layer can be made consistent. For example, the following scheme can be adopted, that is, when a c-plane sapphire substrate is used, the sapphire substrate and the orientation layer are both c-axis oriented relative to the surface of the base substrate. And, through this heat treatment, a gradient composition region can be formed in a part of the orientation layer. That is, during heat treatment, a reaction occurs at the interface between the sapphire substrate and the orientation precursor layer, and the Al component in the sapphire substrate diffuses into the orientation precursor layer and/or the components in the orientation precursor layer diffuse into the _sapphire substrate, forming a gradient composition region composed of a solid solution containing α- _Al2O3 .

It should be noted that it is known that in various methods such as CVD, sputtering, HVPE, PLD, CVT, and sublimation, heteroepitaxial growth sometimes occurs on a sapphire substrate without heat treatment above 1000°C. However, the oriented precursor layer is preferably in an unoriented state, i.e., amorphous or non-oriented polycrystalline, when it is made, and the crystals are rearranged using sapphire as a seed during this heat treatment process. Accordingly, the crystal defects reaching the surface of the oriented layer can be effectively reduced. The reason is uncertain, but it is believed that it is because the crystal defects generated in the lower part of the oriented layer are easily offset.

The heat treatment is not particularly limited as long as it can obtain a corundum-type crystal structure and cause heteroepitaxial growth with a sapphire substrate as a seed, and can be implemented in a known heat treatment furnace such as a tubular furnace or a heating plate. In addition, not only these heat treatments under normal pressure (no pressure) can be used, but also pressurized heat treatments such as hot pressing and HIP, and a combination of normal pressure heat treatment and pressurized heat treatment can be used. The heat treatment conditions can be appropriately selected according to the material used for the orientation layer. For example, the atmosphere for the heat treatment can be selected from air, vacuum, nitrogen and an inert gas atmosphere. The preferred heat treatment temperature also varies depending on the material used for the orientation layer, but, for example, it is preferably 1000 to 2000°C, and more preferably 1200 to 2000°C. The heat treatment temperature and holding time are related to the thickness of the orientation layer produced in the heteroepitaxial growth and the thickness of the gradient composition region formed by diffusion with the sapphire substrate, and can be appropriately adjusted according to the type of material, the target orientation layer, the thickness of the gradient composition region, etc. However, when the prefabricated molded body is used as an oriented precursor layer, it is necessary to sinter it during heat treatment to densify it, preferably normal pressure sintering, hot pressing, HIP, or a combination thereof at high temperature. For example, when hot pressing is used, the surface pressure is preferably above 50kgf/ ^cm2 , more preferably above 100kgf/ ^cm2 , and particularly preferably above 200kgf/ ^cm2 , and the upper limit is not particularly limited. In addition, as long as sintering, densification, and heteroepitaxial growth occur, the firing temperature is not particularly limited, preferably above 1000°C, more preferably above 1200°C, further preferably above 1400°C, and particularly preferably above 1600°C. The firing atmosphere can also be selected from air, vacuum, nitrogen, and inert gas atmospheres. Firing fixtures such as outer molds can utilize fixtures made of graphite or alumina.

(d) Exposure of the Orientation Layer Surface

An orientation precursor layer or a surface layer with poor orientation or no orientation may exist or remain on the orientation layer formed near the sapphire substrate by heat treatment. In this case, it is preferred to apply grinding, polishing and other processing to the surface originating from one side of the orientation precursor layer to expose the surface of the orientation layer. Accordingly, a material with excellent orientation is exposed on the surface of the orientation layer, so that the semiconductor layer can be effectively epitaxially grown thereon. The method for removing the orientation precursor layer and the surface layer is not particularly limited, and examples thereof include: a method of grinding and polishing, and a method of ion beam milling. The surface of the orientation layer is preferably polished by polishing using abrasives or chemical mechanical polishing (CMP).

Semiconductor layer

By using the base substrate of the present invention, a semiconductor layer containing a nitride or oxide of a Group 13 element can be formed. The semiconductor layer can be formed by a known method, but preferably any one of various vapor phase film forming methods such as CVD method, HVPE method, sublimation method, MBE method, PLD method and sputtering method, hydrothermal method, Na flux method and the like, and particularly preferably atomized CVD method, hydrothermal method, or HVPE method. The atomized CVD method is described below.

The atomization CVD method is a method in which a raw material solution is atomized or dropletized to generate a spray or droplets, the spray or droplets are transported to a film forming chamber equipped with a substrate using a carrier gas, the spray or droplets are thermally decomposed and chemically reacted in the film forming chamber, a film is formed and grown on the substrate, and the atomization CVD method does not require a vacuum process and can produce a large number of samples in a short time. Here, an example of an atomization CVD device is shown in FIG2 . The atomization CVD device 61 shown in FIG2 comprises: a susceptor 70 on which a substrate 69 is placed; a dilution gas source 62a; a carrier gas source 62b; a flow rate regulating valve 63a for regulating the flow rate of the dilution gas sent from the dilution gas source 62a; a flow rate regulating valve 63b for regulating the flow rate of the carrier gas sent from the carrier gas source 62b; an atomization source 64 for storing a raw material solution 64a; a container 65 in which water 65a is placed; an ultrasonic oscillator 66 mounted on the bottom surface of the container 65; a quartz tube 67 forming a film forming chamber; a heater 68 provided at the peripheral portion of the quartz tube 67; and an exhaust port 71. The susceptor 70 is made of quartz, and the surface on which the substrate 69 is placed is inclined relative to the horizontal plane.

The raw material solution 64a used in the atomized CVD method may be a solution for obtaining a semiconductor layer containing a nitride or oxide of a Group 13 element, and is not limited thereto. For example, a solution obtained by dissolving Ga and/or an organic metal complex or halide of a metal that forms a solid solution with Ga in a solvent may be cited. As an example of an organic metal complex, an acetylacetone complex may be cited. In addition, when a dopant is added to the semiconductor layer, a solution of the dopant component may be added to the raw material solution. In addition, an additive such as hydrochloric acid may be added to the raw material solution. As a solvent, water, alcohol, etc. may be used.

Next, the obtained raw material solution 64a is atomized or dropletized to generate a spray or droplets 64b. As a preferred example of a method for atomization or dropletization, a method of vibrating the raw material solution 64a using an ultrasonic oscillator 66 can be cited. Then, the obtained spray or droplets 64b are transported to the film forming chamber using a carrier gas. The carrier gas is not particularly limited, and one or more of an inert gas such as oxygen, ozone, nitrogen, and a reducing gas such as hydrogen can be used.

The film forming chamber (quartz tube 67) is provided with a substrate 69. The spray or droplets 64b transported to the film forming chamber undergo thermal decomposition and chemical reaction, thereby forming a film on the substrate 69. The reaction temperature varies depending on the type of raw material solution, and is preferably 300 to 800° C., and more preferably 400 to 700° C. In addition, the atmosphere in the film forming chamber is not particularly limited as long as the desired semiconductor film is obtained, and can be an oxygen atmosphere, an inert gas atmosphere, a vacuum or a reducing atmosphere, and is preferably an air atmosphere.

For a semiconductor layer made using a base substrate like this, the typical surface crystal defect density is significantly low, less than 1.0×10 ⁶ /cm ^2. A semiconductor layer with such a significantly low crystal defect density has excellent dielectric breakdown electric field characteristics and is suitable for use in power semiconductors. It should be noted that the crystal defect density of the semiconductor layer can be evaluated by using a planar TEM observation (top view) or a cross-sectional TEM observation using a conventional transmission electron microscope (TEM). For example, when a transmission electron microscope is used for top view observation using Hitachi H-90001UHR-I, an acceleration voltage of 300 kV is sufficient. A test piece is cut out in a manner that includes the film surface, and ion milling is performed to form a test piece with a measurement field of view of 50μm×50μm and a test piece thickness of 150nm around the measurement field of view. Prepare 10 test pieces like this, observe the TEM images of a total of 10 fields of view, and the crystal defect density can be evaluated with good accuracy. The crystal defect density is preferably 1.0×10 ⁵ /cm ² or less, more preferably 4.0×10 ³ /cm ² or less, and there is no particular lower limit. In addition, by ion milling so that 8 or more measurement fields of 4.1 μm×3.1 μm can be observed and the thickness around the measurement field is 150 nm, the crystal defect density can also be evaluated with good accuracy by observing 8 or more measurement fields of 4.1 μm×3.1 μm. The crystal defect density is preferably 1.0×10 ⁷ /cm ² or less, more preferably 1.0×10 ⁶ /cm ² or less, and further preferably 4.0×10 ³ /cm ² or less, and there is no particular lower limit.

As far as the inventors of the present invention know, there is no known technology that can obtain a semiconductor layer with such a low crystal defect density. For example, Non-Patent Document 1 discloses forming an α-Ga ₂ O ₃ layer using a substrate having an (Al _x , Ga _1-x ) ₂ O ₃ layer (x=0.2 to 0.9) introduced as a buffer layer between sapphire and the α-Ga ₂ O ₃ layer, but the edge dislocation and screw dislocation densities of the obtained α-Ga ₂ O ₃ layer are 3×10 ⁸ /cm ² and 6×10 ⁸ /cm ^2, respectively.

Example

The present invention is further specifically described through the following examples.

example 1

(1) Fabrication of composite base substrate

(1a) Preparation of oriented precursor layer

As a raw material powder, a powder obtained by mixing commercially available Cr ₂ O ₃ powder and commercially available Fe ₂ O ₃ powder at a molar ratio of 72:28 was used, and as a substrate, sapphire (diameter 50.8 mm (2 inches), thickness 0.43 mm, c-plane, off-angle 0.3°) was used, and an AD film (oriented precursor layer) composed of Cr ₂ O ₃ was formed on the seed substrate (sapphire substrate) using the aerosol deposition (AD) device 20 shown in Figure 1. The structure of the aerosol deposition (AD) device 20 is as described above.

The AD film forming conditions are as follows. That is, the carrier gas is set to Ar, and a ceramic nozzle with a slit of 5 mm long side × 0.3 mm short side is used. The scanning conditions of the nozzle are as follows: at a scanning speed of 0.5 mm/s, move 55 mm in the direction perpendicular to the long side of the slit and in the forward direction, move 5 mm in the direction of the long side of the slit, move 55 mm in the direction perpendicular to the long side of the slit and in the return direction, move 5 mm in the direction of the long side of the slit and in the opposite direction to the initial position, repeat this scanning, and at the moment when the long side of the slit has moved 55 mm from the initial position, scan in the opposite direction to the previous direction and return to the initial position. Such a cycle is set as 1 cycle, and 500 cycles are repeated. In the film formation of 1 cycle at room temperature, the set pressure of the transport gas is adjusted to 0.07 MPa, the flow rate is adjusted to 8 L/min, and the pressure in the chamber is adjusted to less than 100 Pa. The AD film (oriented precursor layer) formed in this way has a thickness of 120 μm.

(1b) Heat treatment of the orientation precursor layer

The sapphire substrate on which the AD film (alignment precursor layer) was formed was taken out from the AD device and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

(1c) Grinding and Lapping

The obtained substrate is fixed on a ceramic platform, and a grindstone with a particle size of less than #2000 is used to grind the surface originating from the AD film side until the orientation layer is exposed, and then the plate surface is further smoothed by grinding using diamond abrasives. At this time, the grinding process is performed while the size of the diamond abrasive is gradually reduced from 3μm to 0.5μm, thereby improving the flatness of the plate surface. Then, mirror finishing is performed using chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate having an orientation layer on a sapphire substrate. The arithmetic mean roughness Ra of the surface of the orientation layer after processing is 0.1nm, the grinding and polishing amount is 70μm, and the thickness of the composite base substrate after grinding is 0.48mm. It should be noted that the surface on the side where the AD film is formed is referred to as the "surface".

(2) Evaluation of Orientation Layer

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis of a cross section perpendicular to the main surface of the substrate. As a result, only Cr, Fe, and O were detected in the range from the surface of the composite base substrate to a depth of 20 μm. The ratio of Cr, Fe, and O hardly changed in the range of 20 μm in depth, indicating that a 20 μm thick Cr-Fe oxide layer (composition stable region) was formed. In addition, Cr, Fe, O, and Al were detected in the range from the Cr-Fe oxide layer to a depth of 60 μm, indicating that a 60 μm thick Cr-Fe-Al oxide layer (gradient composition layer) was formed between the Cr-Fe oxide layer and the sapphire substrate. In the Cr-Fe-Al oxide layer, the following situation was confirmed, that is, the ratio of Al to Cr and Fe was different, the Al concentration was higher on the sapphire substrate side, and the Al concentration was lower on the side close to the Cr-Fe oxide layer. From the above, it can be seen that the thickness of the composition stable region is 20 μm, the thickness of the gradient composition layer is 60 μm, and the thickness of the entire orientation layer is 80 μm. The thickness of the composite base substrate is 0.48 mm, which means that the thickness of the sapphire substrate in the composite base substrate is 0.40 mm.

(2b) Surface EBSD

The inverse pole figure orientation mapping of the surface of the oriented layer composed of the Cr oxide layer was performed in a field of view of 500 μm×500 μm using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction device (EBSD) (Nordlys Nano manufactured by Oxford Instruments). The conditions for the EBSD measurement are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Fe oxide layer is a layer with a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal to the c-axis and also oriented in the in-plane direction. This shows that an oriented layer composed of a solid solution of α-Cr ₂ O ₃ and α-Fe ₂ O ₃ is formed on the substrate surface. Based on the above results, the production process of the composite base substrate is schematically shown in Figures 3(a) to (d).

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed using a multifunctional high-resolution X-ray diffractometer (D8DISCOVER manufactured by Bruker AXS Co., Ltd.). Specifically, after adjusting the Z axis according to the height of the substrate surface, the axes of χ, φ, ω, and 2θ were adjusted relative to the (11-20) crystal plane of the corundum type crystal structure, and the 2θ-ω measurement was performed under the following conditions.

＜XRD measurement conditions＞

Tube voltage: 40kV

Tube current: 40mA

Detector: Tripple Ge(220)Analyzer

·CuKα radiation obtained by parallel monochromation (half-value width 28 seconds) using Ge(022) asymmetric reflection monochromator

Step width: 0.001°

Scanning speed: 1.0 sec/step

The results show that the length of the a-axis on the surface of the orientation layer is

(2d) Evaluation of the back side of the orientation layer (sapphire substrate side)

In the same manner as in (1) above, a composite base substrate is prepared separately. The surface (orientation layer side) of the obtained composite base substrate is bonded to another sapphire substrate, and the 0.40 mm thick sapphire substrate disposed on the back side of the composite base substrate is removed by grinding to expose the back side of the orientation layer. Next, a grindstone is used to grind the surface after the sapphire substrate is removed (the back side of the orientation layer) to #2000 to make the plate surface flat. Next, the plate surface is smoothed by grinding with diamond abrasives. At this time, the grinding process is performed while the size of the diamond abrasives is gradually reduced from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. Then, a mirror finish is performed by chemical mechanical polishing (CMP) using colloidal silica to prepare a sample for evaluating the back side of the orientation layer. Accordingly, the preparation process of the sample for evaluating the back side of the orientation layer is schematically shown in Figures 4 (a) to (c).

The EBSD measurement of the back side of the orientation layer was carried out in the same manner as in (2b) above. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Fe-Al oxide layer constituting the back side of the orientation layer is a layer having a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the in-plane direction. The back side of the orientation layer is a composition gradient layer, and therefore, it can be seen that the composition gradient layer is composed of a solid solution of Cr ₂ O ₃ , Fe ₂ O ₃ , and Al ₂ O ₃ .

Next, the XRD in-plane measurement of the back side of the orientation layer was performed in the same manner as in (2c) above. The results showed that the back side of the orientation layer also belonged to a biaxially oriented single-phase corundum material, with an a-axis length of This shows that the a-axis length on the surface of the orientation layer is longer than that on the back side of the orientation layer (i.e., [{(a-axis length on the surface of the orientation layer) - (a-axis length on the back side of the orientation layer)}/(a-axis length on the back side of the orientation layer)]×100=4.6%).

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

Using the mist CVD apparatus 61 shown in FIG. 2 , an α-Ga ₂ O ₃ film was formed on the surface of the orientation layer of the obtained composite base substrate as follows.

(3a) Preparation of raw material solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared, and 1.8% by volume of 38% hydrochloric acid was contained to prepare a raw material solution 64a.

(3b) Film preparation

The obtained raw material solution 64a is stored in the atomization source 64. The composite base substrate prepared in the above (1) is set as a substrate 69 on the base 70, and the heater 68 is operated to raise the temperature in the quartz tube 67 to 610°C. Next, the flow control valves 63a and 63b are opened, and the dilution gas and the carrier gas are respectively supplied from the dilution gas source 62a and the carrier gas source 62b to the quartz tube 67. After the atmosphere of the quartz tube 67 is fully replaced with the dilution gas and the carrier gas, the flow rate of the dilution gas is adjusted to 0.6L/min, and the flow rate of the carrier gas is adjusted to 1.2L/min. Nitrogen gas is used as the dilution gas and the carrier gas.

(3c) Film formation

The ultrasonic oscillator 66 is vibrated at 2.4 MHz, and the vibration is transmitted to the raw material solution 64a through the water 65a, thereby atomizing the raw material solution 64a to generate a spray 64b. The spray 64b is introduced into the quartz tube 67 as a film forming chamber through the dilution gas and the carrier gas, and reacts in the quartz tube 67 to form a film on the substrate 69 through the CVD reaction on the surface of the substrate 69. In this way, a crystalline semiconductor film (semiconductor layer) is obtained. The film formation time is 60 minutes.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction (EBSD) device (Nordlys Nano manufactured by Oxford Instruments), inverse pole figure orientation mapping of the Ga oxide film surface was performed within a field of view of 500 μm×500 μm. The EBSD measurement conditions are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

The obtained inverse pole figure orientation mapping revealed that the Ga oxide film had a biaxially oriented corundum-type crystal structure with c-axis orientation in the substrate normal direction and also in-plane orientation, which indicated that an oriented film composed of α-Ga ₂ O ₃ was formed.

(4c) Planar TEM of the film-forming side surface

In order to evaluate the crystal defect density of the α-Ga ₂ O ₃ film, a planar TEM observation (top view) was performed. The film was cut in a manner that included the surface of the film-forming side, and the sample thickness (T) around the measurement field of view was processed by ion milling to be 150 nm. The obtained slices were subjected to TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an accelerating voltage of 300 kV to evaluate the crystal defect density. In fact, TEM images of a measurement field of view of 4.1μm×3.1μm were observed in 8 fields of view. As a result, no crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least less than 9.9×10 ⁵ /cm ² .

Example 2

(1) Fabrication of composite base substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing commercially available Cr ₂ O ₃ powder, commercially available Fe ₂ O ₃ powder and commercially available Al ₂ O ₃ powder at a molar ratio of 45:45:10 was used as a raw material powder of the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis of a cross section orthogonal to the main surface of the substrate. As a result, only Cr, Fe, Al and O were detected in the range from the surface of the composite base substrate to a depth of 20 μm. The ratio of Cr, Fe, Al and O hardly changed in the range up to a depth of 20 μm, and it was known that a Cr-Fe-Al oxide layer (composition stable region) with a thickness of 20 μm was formed. In addition, it was confirmed that Cr, Fe, O and Al were also detected in the range from the Cr-Fe-Al oxide layer to a depth of 60 μm, but in this region, the ratio of Al to Cr and Fe was different. On the sapphire substrate side, the Al concentration was higher, and on the side close to the composition stable region, the Al concentration decreased. Therefore, it can be seen that this range is a composition gradient region. From the above, it can be seen that the thickness of the composition stable region is 20 μm, the thickness of the gradient composition layer is 60 μm, and the thickness of the entire orientation layer is 80 μm. The thickness of the composite base substrate is 0.48 mm, which means that the thickness of the sapphire substrate in the composite base substrate is 0.40 mm.

(2b) Surface EBSD

Similar to Example 1 (2b), the inverse pole figure orientation mapping of the substrate surface composed of the Cr-Fe-Al oxide layer was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Fe-Al oxide layer is a layer having a biaxially oriented corundum-type crystal structure that is oriented in the c-axis direction in the normal direction of the substrate and also oriented in the in-plane direction. This shows that an oriented layer composed of a solid solution of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , and α-Al ₂ O ₃ is formed on the substrate surface.

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed in the same manner as in Example 1 (2c). The results showed that the a-axis length of the orientation layer surface was

(2d) Evaluation of the back side of the orientation layer (sapphire substrate side)

In the same manner as in (1) above, a composite base substrate is separately prepared, and then, a sample for evaluating the back side of the orientation layer is prepared in the same order as in Example 1 (2d). EBSD measurement of the back side of the orientation layer is carried out in the same manner as in (2b) above. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Fe-Al oxide layer constituting the back side of the orientation layer is a layer having a biaxially oriented corundum-type crystal structure that is c-axis oriented in the normal direction of the substrate and also oriented in the in-plane direction. The back side of the orientation layer belongs to a composition gradient layer, and therefore, it can be seen that the composition gradient layer is composed of a solid solution of Cr ₂ O ₃ , Fe ₂ O ₃ , and Al ₂ O _3. In addition, in the same manner as in (2c) above, XRD in-plane measurement of the back side of the orientation layer is carried out. The results show that the back side of the orientation layer also belongs to a biaxially oriented single-phase corundum material with an a-axis length of This shows that the a-axis length on the surface of the orientation layer is longer than that on the back side of the orientation layer (i.e., [{(a-axis length on the surface of the orientation layer) - (a-axis length on the back side of the orientation layer)}/(a-axis length on the back side of the orientation layer)]×100=4.6%).

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

In the same manner as in Example 1 (3), an α-Ga ₂ O ₃ film was formed on the composite base substrate.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Similar to Example 1 (4b), the inverse pole figure orientation mapping of the film surface composed of Ga oxide was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Ga oxide film has a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the plane. This indicates that an oriented film composed of α-Ga ₂ O ₃ is formed.

(4c) Planar TEM of the film-forming side surface

The crystal defect density of the film-forming side surface of the α-Ga ₂ O ₃ film was evaluated in the same manner as in Example 1 (4c). As a result, no crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least less than 9.9×10 ⁵ /cm ² .

Example 3

(1) Fabrication of base substrate

(1a) Fabrication of composite base substrate

A composite base substrate was prepared in the same manner as in Example 1(1).

(1b) Thickening of the Orientation Layer

To thicken the alignment layer, an AD film (alignment precursor layer) is formed again on the alignment layer of the composite base substrate. Using the AD film forming apparatus 20 shown in FIG. 1 , an AD film (alignment precursor layer) composed of Cr ₂ O ₃ and Fe ₂ O ₃ is formed on the alignment layer of the composite base substrate.

The AD film forming conditions are as follows. That is, the carrier gas is set to Ar, and a ceramic nozzle with a slit of 5 mm long side × 0.3 mm short side is used. The scanning conditions of the nozzle are as follows: at a scanning speed of 0.5 mm/s, move 55 mm in the direction perpendicular to the long side of the slit and forward, move 5 mm in the direction perpendicular to the long side of the slit and return, move 55 mm in the direction perpendicular to the long side of the slit and in the direction opposite to the initial position, and repeat this scanning. At the moment when the long side of the slit has moved 55 mm from the initial position, scan in the opposite direction to the previous direction and return to the initial position. Such a cycle is set as 1 cycle, and repeated 500 cycles. In the film formation of 1 cycle at room temperature, the set pressure of the transport gas is adjusted to 0.07 MPa, the flow rate is adjusted to 8 L/min, and the pressure in the chamber is adjusted to less than 100 Pa. The AD film formed in this way has a thickness of 120 μm.

The composite base substrate on which the AD film was formed was taken out from the AD device and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

The obtained substrate is fixed on a ceramic platform, and a grindstone with a particle size of less than #2000 is used to grind the surface from one side of the AD film until the orientation layer is exposed. Then, the plate surface is further smoothed by grinding with diamond abrasives. At this time, the size of the diamond abrasive is gradually reduced from 3μm to 0.5μm while grinding, thereby improving the flatness of the plate surface. Then, chemical mechanical polishing (CMP) using colloidal silica is used to perform mirror finishing to obtain a composite base substrate with an orientation layer on a sapphire substrate. The arithmetic mean roughness Ra of the surface of the processed orientation layer is 0.1nm, the grinding and polishing amount is 50μm, and the thickness of the substrate after grinding is 0.48mm.

The series of steps for thickening (i.e., AD film formation - annealing - grinding and polishing) were repeated 9 times in total (i.e., if the preparation sequence of the composite base substrate in (1a) is counted as 1 time, the series of steps were repeated 10 times in total). Result: The thickness of the composite base substrate after the final grinding was 0.93 mm. It should be noted that the surface on the side where the AD film is formed is referred to as the "surface".

(1c) Self-sustaining Orientation Layer

The substrate thus obtained is fixed on a ceramic platform, and a grindstone is used to grind the surface (back side) opposite to the surface (surface) originating from the AD film side, that is, the surface on the sapphire substrate side, to remove the sapphire substrate. Then, a grindstone is used to grind the surface of the orientation layer on the sapphire substrate side to #2000, so that the plate surface becomes flat. Next, the plate surface is smoothed by grinding with diamond abrasives. At this time, the grinding process is performed while the size of the diamond abrasives is gradually reduced from 3μm to 0.5μm, thereby improving the flatness of the plate surface. The grinding and polishing amount including the sapphire is 480μm, and the thickness of the substrate after grinding is 0.45mm. The obtained substrate is a self-supporting substrate consisting only of an orientation layer.

(2) Evaluation of self-supporting substrates

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis on a cross section perpendicular to the main surface of the substrate. As a result, only Cr, Fe, and O were detected in the entire cross section of the self-supporting substrate. The ratio of Cr, Fe, and O hardly changed in the entire area, indicating that the self-supporting substrate is composed of a single-phase Cr-Fe oxide.

(2b) Surface EBSD

Using an SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction device (EBSD) (Nordlys Nano manufactured by Oxford Instruments), inverse pole figure orientation mapping of the surface (the surface on the side where the AD film is formed) and the back side (the surface on the side in contact with the sapphire substrate) of the substrate composed of a Cr oxide layer was performed within a field of view of 500μm×500μm. The EBSD measurement was performed under the same conditions as Example 1 (2b). From the obtained inverse pole figure orientation mapping, it can be seen that the surface and back side of the self-supporting substrate are composed of a biaxially oriented corundum-type crystal structure that is c-axis oriented in the normal direction of the substrate and also oriented in the in-plane direction. This shows that the surface and back side of the self-supporting substrate are composed of a biaxially oriented solid solution of α-Cr ₂ O ₃ and α-Fe ₂ O ₃ .

(2c)XRD

The XRD in-plane measurement of the front and back surfaces of the self-supporting substrate was performed in the same manner as in Example 1 (2c). The results showed that the a-axis lengths of the front and back surfaces of the self-supporting substrate were both

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

In the same manner as in Example 1 (3), an α-Ga ₂ O ₃ film was formed on a self-supporting substrate.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Similar to Example 1 (4b), the inverse pole figure orientation mapping of the film surface composed of Ga oxide was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Ga oxide film has a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the plane. This indicates that an oriented film composed of α-Ga ₂ O ₃ is formed.

(4c) Planar TEM of the film-forming side surface

The crystal defect density of the film-forming side surface of the α-Ga ₂ O ₃ film was evaluated in the same manner as in Example 1 (4c). As a result, no crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least less than 9.9×10 ⁵ /cm ² .

Example 4

(1) Fabrication of composite base substrate

(1a) Preparation of oriented precursor layer

Commercially available Cr ₂ O ₃ powder was used as a raw material powder, and an AD film (orientation precursor layer) composed of Cr ₂ O ₃ was formed on a seed substrate (sapphire substrate) using the AD device 20 shown in FIG. 1 .

The AD film forming conditions are as follows. That is, the carrier gas is set to Ar, and a ceramic nozzle with a slit of 5 mm long side × 0.3 mm short side is used. The scanning conditions of the nozzle are as follows: at a scanning speed of 0.5 mm/s, move 55 mm in the direction perpendicular to the long side of the slit and forward, move 5 mm in the direction perpendicular to the long side of the slit and return, move 55 mm in the direction perpendicular to the long side of the slit and in the direction opposite to the initial position, and repeat this scanning. At the moment when the long side of the slit has moved 55 mm from the initial position, scan in the opposite direction to the previous direction and return to the initial position. Such a cycle is set as 1 cycle, and repeated 500 cycles. In the film formation of 1 cycle at room temperature, the set pressure of the transport gas is adjusted to 0.07 MPa, the flow rate is adjusted to 8 L/min, and the pressure in the chamber is adjusted to less than 100 Pa. The AD film formed in this way has a thickness of 120 μm.

(1b) Heat treatment of the orientation precursor layer

The sapphire substrate on which the AD film was formed was taken out from the AD device and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

(1c) Grinding and Lapping

The obtained substrate is fixed on a ceramic platform, and a grindstone with a particle size of less than #2000 is used to grind the surface from the AD film side until the orientation layer is exposed, and then the plate surface is further smoothed by grinding with diamond abrasives. At this time, the grinding process is performed while the size of the diamond abrasive is gradually reduced from 3μm to 0.5μm, thereby improving the flatness of the plate surface. Then, mirror finishing is performed using chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate with an orientation layer on a sapphire substrate. The arithmetic mean roughness Ra of the processed orientation layer surface is 0.1nm, the grinding and polishing amount is 100μm, and the thickness of the composite base substrate after grinding is 0.45mm. It should be noted that the surface on the side where the AD film is formed is referred to as the "surface".

(2) Evaluation of Orientation Layer

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis of a cross section orthogonal to the main surface of the substrate. As a result, Cr, O, and Al were detected in the range from the surface of the composite base substrate to a depth of 50 μm. It was found that a 50 μm Cr-Al oxide layer (gradient composition layer) was formed between the surface and the sapphire substrate. In the Cr-Al oxide layer, the following situation was confirmed, that is, the ratio of Cr and Al was different, the Al concentration was higher on the sapphire substrate side, and the Al concentration was lower on the side close to the surface. From the above, it can be seen that the overall thickness of the orientation layer is 50 μm. The thickness of the composite base substrate is 0.45 mm, which shows that the thickness of the sapphire substrate in the composite base substrate is 0.40 mm.

(2b) Surface EBSD

Using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction device (EBSD) (Nordlys Nano manufactured by Oxford Instruments), the reverse pole figure orientation mapping of the substrate surface composed of the Cr-Al oxide layer was performed in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

The obtained inverse pole figure orientation mapping shows that the Cr-Al oxide layer is a layer with a biaxially oriented corundum-type crystal structure that is oriented in the c-axis direction in the normal direction of the substrate and also oriented in the in-plane direction. This indicates that an oriented layer composed of a solid solution of α-Cr ₂ O ₃ and α-Al ₂ O ₃ is formed on the substrate surface.

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed using a multifunctional high-resolution X-ray diffractometer (D8DISCOVER manufactured by Bruker AXS Co., Ltd.). Specifically, after adjusting the Z axis according to the height of the substrate surface, the axes of χ, φ, ω, and 2θ were adjusted relative to the (11-20) crystal plane, and the 2θ-ω measurement was performed under the following conditions.

＜XRD measurement conditions＞

Tube voltage: 40kV

Tube current: 40mA

Detector: Tripple Ge(220)Analyzer

·CuKα radiation obtained by parallel monochromation (half-value width 28 seconds) using Ge(022) asymmetric reflection monochromator

Step width: 0.001°

Scanning speed: 1.0 sec/step

The results show that the length of the a-axis on the surface of the orientation layer is

(2d) Evaluation of the back side of the orientation layer (sapphire substrate side)

In the same manner as in (1) above, a composite base substrate is prepared separately. The surface (orientation layer side) of the obtained composite base substrate is bonded to another sapphire substrate, and the 0.4 mm thick sapphire substrate disposed on the back side of the composite base substrate is removed by grinding to expose the back side of the orientation layer. Next, a grindstone is used to grind the surface after the sapphire substrate is removed (the back side of the orientation layer) to #2000 to make the plate surface flat. Next, the plate surface is smoothed by grinding with diamond abrasives. At this time, the grinding process is performed while the size of the diamond abrasives is gradually reduced from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. Then, mirror finishing is performed by chemical mechanical polishing (CMP) using colloidal silica to prepare a sample for evaluating the back side of the orientation layer.

The EBSD measurement of the back side of the orientation layer was carried out in the same manner as in (2b) above. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Al oxide layer constituting the back side of the orientation layer is a layer having a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the in-plane direction. The back side of the orientation layer belongs to the composition gradient layer, and therefore, it can be seen that the composition gradient layer is composed of a solid solution of Cr ₂ O ₃ and Al ₂ O ₃ .

Next, the XRD in-plane measurement of the back side of the orientation layer was performed in the same manner as in (2c) above. The results showed that the back side of the orientation layer also belonged to a biaxially oriented single-phase corundum material, with an a-axis length of This shows that the a-axis length on the surface of the orientation layer is longer than that on the back side of the orientation layer, and the difference between the a-axis lengths on the surface and back sides (=[{(a-axis length on the surface) - (a-axis length on the back side)}/(a-axis length on the back side)]×100) is 2.9%.

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

Using the mist CVD apparatus 61 shown in FIG. 2 , an α-Ga ₂ O ₃ film was formed on the surface of the orientation layer of the obtained composite base substrate as follows.

(3a) Preparation of raw material solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared, and 1.8% by volume of 38% hydrochloric acid was contained to prepare a raw material solution 64a.

(3b) Film preparation

The obtained raw material solution 64a is stored in the atomization source 64. The composite base substrate prepared in the above (1) is set as a substrate 69 on the base 70, and the heater 68 is operated to raise the temperature in the quartz tube 67 to 610°C. Next, the flow control valves 63a and 63b are opened, and the dilution gas and the carrier gas are respectively supplied from the dilution gas source 62a and the carrier gas source 62b to the quartz tube 67. After the atmosphere of the quartz tube 67 is fully replaced with the dilution gas and the carrier gas, the flow rate of the dilution gas is adjusted to 0.6L/min, and the flow rate of the carrier gas is adjusted to 1.2L/min. Nitrogen gas is used as the dilution gas and the carrier gas.

(3c) Film formation

The ultrasonic oscillator 66 is vibrated at 2.4 MHz, and the vibration is transmitted to the raw material solution 64a through the water 65a, thereby atomizing the raw material solution 64a to generate a spray 64b. The spray 64b is introduced into the quartz tube 67 as a film forming chamber through the dilution gas and the carrier gas, and reacts in the quartz tube 67 to form a film on the substrate 69 through the CVD reaction on the surface of the substrate 69. In this way, a crystalline semiconductor film (semiconductor layer) is obtained. The film formation time is 60 minutes.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction (EBSD) device (Nordlys Nano manufactured by Oxford Instruments), inverse pole figure orientation mapping of the Ga oxide film surface was performed within a field of view of 500 μm×500 μm. The EBSD measurement conditions are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

The obtained inverse pole figure orientation mapping revealed that the Ga oxide film had a biaxially oriented corundum-type crystal structure with c-axis orientation in the substrate normal direction and also in-plane orientation, which indicated that an oriented film composed of α-Ga ₂ O ₃ was formed.

(4c) Planar TEM of the film-forming side surface

In order to evaluate the crystal defect density of the α-Ga ₂ O ₃ film, a planar TEM observation (top view) was performed. The film was cut in a manner that included the surface of the film-forming side, and the sample thickness (T) around the measurement field of view was processed by ion milling to be 150 nm. The obtained slices were subjected to TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an acceleration voltage of 300 kV to evaluate the crystal defect density. In fact, TEM images with a measurement field of view of 4.1μm×3.1μm were observed in 8 fields of view. As a result, in the obtained TEM image, based on the number of crystal defects, it can be seen that the crystal defect density is 9.4×10 ⁶ /cm ² .

Example 5

(1) Fabrication of composite base substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing commercially available Cr ₂ O ₃ powder and commercially available TiO ₂ powder at a molar ratio of 97:3 was used as a raw material powder of the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis of a cross section orthogonal to the main surface of the substrate. As a result, only Cr, Ti, and O were detected in the range from the surface of the composite base substrate to a depth of 20 μm. The ratio of Cr, Ti, and O hardly changed in the range of 20 μm, indicating that a 20 μm thick Cr-Ti oxide layer (composition stable region) was formed. In addition, Cr, Ti, O, and Al were detected in the range from the Cr-Ti oxide layer to a depth of 60 μm, indicating that a 60 μm Cr-Ti-Al oxide layer (gradient composition layer) was formed between the Cr-Ti oxide layer and the sapphire substrate. In the Cr-Ti-Al oxide layer, the following situation was confirmed, that is, the ratio of Al to Cr and Ti was different, the Al concentration was higher on the sapphire substrate side, and the Al concentration was lower on the side close to the Cr-Ti oxide layer. From the above, it can be seen that the thickness of the composition stable region is 20 μm, the thickness of the gradient composition layer is 60 μm, and the thickness of the orientation layer as a whole is 80 μm. The thickness of the composite base substrate is 0.48 mm, which means that the thickness of the sapphire substrate in the composite base substrate is 0.40 mm.

(2b) Surface EBSD

Similar to Example 1 (2b), the inverse pole figure orientation mapping of the substrate surface composed of the Cr-Ti oxide layer was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Ti oxide layer is a layer having a biaxially oriented corundum-type crystal structure that is oriented in the c-axis direction in the normal direction of the substrate and also oriented in the in-plane direction. This shows that an oriented layer composed of a material in which the Ti component is solid-dissolved in α-Cr ₂ O ₃ is formed on the substrate surface.

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed in the same manner as in Example 1 (2c). The results showed that the a-axis length of the orientation layer surface was

(2d) Evaluation of the back side of the orientation layer (sapphire substrate side)

In the same manner as in (1) above, a composite base substrate is separately prepared, and then, a sample for evaluating the back side of the orientation layer is prepared in the same order as in Example 1 (2d). EBSD measurement of the back side of the orientation layer is carried out in the same manner as in (2b) above. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Ti-Al oxide layer constituting the back side of the orientation layer is a layer having a biaxially oriented corundum-type crystal structure that is c-axis oriented in the normal direction of the substrate and also oriented in the in-plane direction. The back side of the orientation layer belongs to a composition gradient layer, and therefore, it can be seen that the composition gradient layer is composed of a solid solution of Cr ₂ O ₃ , Ti components, and Al ₂ O _3. In addition, in the same manner as in (2c) above, XRD in-plane measurement of the back side of the orientation layer is carried out. The results show that the back side of the orientation layer also belongs to a biaxially oriented single-phase corundum material with an a-axis length of This shows that the a-axis length on the surface of the orientation layer is longer than that on the back side of the orientation layer, and the difference between the a-axis lengths on the surface and back sides (=[{(a-axis length on the surface) - (a-axis length on the back side)}/(a-axis length on the back side)]×100) is 4.5%.

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

In the same manner as in Example 1 (3), an α-Ga ₂ O ₃ film was formed on the composite base substrate.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Similar to Example 1 (4b), the inverse pole figure orientation mapping of the film surface composed of Ga oxide was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Ga oxide film has a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the plane. This indicates that an oriented film composed of α-Ga ₂ O ₃ is formed.

(4c) Planar TEM of the film-forming side surface

In order to evaluate the crystal defect density of the α-Ga ₂ O ₃ film, a planar TEM observation (top view) was performed. The film was cut in a manner that included the surface of the film-forming side, and the sample thickness (T) around the measurement field of view was processed by ion milling to be 150 nm. The obtained slices were subjected to TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an accelerating voltage of 300 kV to evaluate the crystal defect density. In fact, TEM images of a measurement field of view of 4.1μm×3.1μm were observed in 8 fields of view. As a result, no crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least less than 9.9×10 ⁵ /cm ² .

Example 6

(1) Fabrication of composite base substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing commercially available Cr ₂ O ₃ powder, commercially available TiO ₂ powder, and commercially available Fe ₂ O ₃ powder in a molar ratio of 82:2:16 was used as a raw material powder of the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-sectional EDX

An energy dispersive X-ray analyzer (EDX) was used to perform composition analysis of a cross section orthogonal to the main surface of the substrate. As a result, only Cr, Ti, Fe, and O were detected in the range from the surface of the composite base substrate to a depth of 20 μm. The ratio of Cr, Ti, Fe, and O hardly changed in the range of 20 μm, indicating that a Cr-Ti-Fe oxide layer with a thickness of 20 μm (composition stability region) was formed. In addition, Cr, Ti, Fe, O, and Al were detected in the range from the Cr-Ti-Fe oxide layer to a depth of 60 μm, indicating that a 60 μm Cr-Ti-Fe-Al oxide layer (gradient composition layer) was formed between the Cr-Ti-Fe oxide layer and the sapphire substrate. In the Cr-Ti-Fe-Al oxide layer, the following situation was confirmed, that is, the ratio of Al to Cr, Ti, and Fe was different, the Al concentration was higher on the sapphire substrate side, and the Al concentration was lower on the side close to the Cr-Ti-Fe oxide layer. From the above, we can know that the thickness of the stable composition region is 20 μm, the thickness of the gradient composition layer is 60 μm, and the thickness of the entire orientation layer is 80 μm. The thickness of the composite base substrate is 0.48 mm, which means that the thickness of the sapphire substrate in the composite base substrate is 0.40 mm.

(2b) Surface EBSD

Similar to Example 1 (2b), the inverse pole figure orientation mapping of the substrate surface composed of the Cr-Ti-Fe oxide layer was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Ti-Fe oxide layer is a layer having a biaxially oriented corundum-type crystal structure that is oriented in the c-axis direction in the normal direction of the substrate and also oriented in the in-plane direction. This shows that an oriented layer composed of a solid solution of α-Cr ₂ O ₃ , Ti component, and α-Fe ₂ O ₃ is formed on the substrate surface.

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed in the same manner as in Example 1 (2c). The results showed that the a-axis length of the orientation layer surface was

(2d) Evaluation of the back side of the orientation layer (sapphire substrate side)

In the same manner as in (1) above, a composite base substrate is separately prepared, and then, a sample for evaluating the back side of the orientation layer is prepared in the same order as in Example 1 (2d). EBSD measurement of the back side of the orientation layer is carried out in the same manner as in (2b) above. From the obtained inverse pole figure orientation mapping, it can be seen that the Cr-Ti-Fe-Al oxide layer constituting the back side of the orientation layer is a layer having a biaxially oriented corundum-type crystal structure that is c-axis oriented in the normal direction of the substrate and also oriented in the in-plane direction. The back side of the orientation layer belongs to a composition gradient layer, and therefore, it can be seen that the composition gradient layer is composed of a solid solution of Cr ₂ O ₃ , Ti component, Fe ₂ O ₃ , and Al ₂ O _3. In addition, in the same manner as in (2c) above, XRD in-plane measurement of the back side of the orientation layer is carried out. The results show that the back side of the orientation layer also belongs to a biaxially oriented single-phase corundum material with an a-axis length of This shows that the a-axis length on the surface of the orientation layer is longer than that on the back side of the orientation layer, and the difference between the a-axis lengths on the surface and back sides (=[{(a-axis length on the surface) - (a-axis length on the back side)}/(a-axis length on the back side)]×100) is 4.6%.

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

In the same manner as in Example 1 (3), an α-Ga ₂ O ₃ film was formed on the composite base substrate.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Similar to Example 1 (4b), the inverse pole figure orientation mapping of the film surface composed of Ga oxide was performed in a field of view of 500 μm × 500 μm. From the obtained inverse pole figure orientation mapping, it can be seen that the Ga oxide film has a biaxially oriented corundum-type crystal structure that is oriented in the direction of the substrate normal and in the plane. This indicates that an oriented film composed of α-Ga ₂ O ₃ is formed.

(4c) Planar TEM of the film-forming side surface

In order to evaluate the crystal defect density of the α-Ga ₂ O ₃ film, a planar TEM observation (top view) was performed. The film was cut in a manner that included the surface of the film-forming side, and the sample thickness (T) around the measurement field of view was processed by ion milling to be 150 nm. The obtained slices were subjected to TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an accelerating voltage of 300 kV to evaluate the crystal defect density. In fact, TEM images of a measurement field of view of 4.1μm×3.1μm were observed in 8 fields of view. As a result, no crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least less than 9.9×10 ⁵ /cm ² .

Example 7 (Comparison)

(1) Fabrication of base substrate

Using the mist CVD apparatus 61 shown in FIG. 2 , an α-Cr ₂ O ₃ film was formed on the surface of a sapphire substrate (diameter 50.8 mm (2 inches), thickness 0.43 mm, c-plane, off-angle 0.3°) as follows.

(1a) Preparation of raw material solution

An aqueous solution having an ammonium dichromate concentration of 0.1 mol/L was prepared and designated as a raw material solution 64a.

(1b) Film preparation

The obtained raw material solution 64a is stored in the atomization source 64. A sapphire substrate is set as a substrate 69 on a base 70, and the heater 68 is operated to raise the temperature in the quartz tube 67 to 410°C. Next, the flow control valves 63a and 63b are opened, and the dilution gas and the carrier gas are respectively supplied from the dilution gas source 62a and the carrier gas source 62b to the quartz tube 67. After the atmosphere of the quartz tube 67 is fully replaced with the dilution gas and the carrier gas, the flow rate of the dilution gas is adjusted to 2.2L/min, and the flow rate of the carrier gas is adjusted to 4.8L/min. Nitrogen gas is used as the dilution gas and the carrier gas.

(1c) Film formation

The ultrasonic oscillator 66 was vibrated at 2.4 MHz, and the vibration was transmitted to the raw material solution 64a through the water 65a, thereby atomizing the raw material solution 64a to generate a spray 64b. The spray 64b was introduced into the quartz tube 67 as a film forming chamber through the dilution gas and the carrier gas, and reacted in the quartz tube 67 to form a film on the substrate 69 through the CVD reaction on the surface of the substrate 69. The film formation was carried out for 30 minutes to obtain an oxide accumulation layer.

(2) Evaluation of Orientation Layer

(2a) Surface EDX

The composition analysis of the substrate surface was performed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr and O were detected, indicating that the oxide accumulation layer was Cr oxide. It should be noted that due to the thin film thickness, cross-sectional EDX measurement was difficult to perform.

(2b) Surface EBSD

Using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction (EBSD) device (Nordlys Nano manufactured by Oxford Instruments), the reverse pole figure orientation mapping of the substrate surface composed of the Cr oxide layer was performed in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

The obtained inverse pole figure orientation mapping shows that the Cr oxide layer is a layer having a corundum-type crystal structure with c-axis orientation in the substrate normal direction. However, a domain deviating by 60° was confirmed in the in-plane direction. This indicates that a uniaxially oriented layer of α-Cr ₂ O ₃ is formed on the substrate surface.

(2c)XRD

The XRD in-plane measurement of the substrate surface was performed using a multifunctional high-resolution X-ray diffractometer (D8DISCOVER manufactured by Bruker AXS Co., Ltd.). Specifically, after adjusting the Z axis according to the height of the substrate surface, the axes of χ, φ, ω, and 2θ were adjusted relative to the (11-20) crystal plane of the corundum type crystal structure, and the 2θ-ω measurement was performed under the following conditions.

＜XRD measurement conditions＞

Tube voltage: 40kV

Tube current: 40mA

Detector: Tripple Ge(220)Analyzer

·CuKα radiation obtained by parallel monochromation (half-value width 28 seconds) using Ge(022) asymmetric reflection monochromator

Step width: 0.001°

Scanning speed: 1.0 sec/step

The results show that the length of the a-axis on the surface of the orientation layer is

(2d) Cross-sectional TEM observation of the oriented layer

In order to evaluate the change in the lattice constant of the α-Cr ₂ O ₃ film, cross-sectional TEM observation was performed. A cross-sectional sample was cut in a manner that included the surface and back side of the film formation side (interface on the sapphire side), and was processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained slice was subjected to cross-sectional TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an accelerating voltage of 300 kV to obtain electron beam diffraction images of the surface and back side of the α-Cr ₂ O ₃ film. The electron beam incident direction was Cr ₂ O ₃ <1-10>. Result: No difference was confirmed in the electron beam diffraction images of the film surface and the film back side, indicating that there was no change in the lattice constant on the surface and back side of the α-Cr ₂ O ₃ film.

(3) Formation of α-Ga ₂ O ₃ film using atomized CVD

Using the mist CVD apparatus 61 shown in FIG. 2 , an α-Ga ₂ O ₃ film was formed on the surface of the obtained base substrate as follows.

(3a) Preparation of raw material solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared, and 1.8% by volume of 38% hydrochloric acid was contained to prepare a raw material solution 64a.

(3b) Film preparation

The obtained raw material solution 64a is stored in the atomization source 64. The composite base substrate prepared in the above (1) is set as a substrate 69 on the base 70, and the heater 68 is operated to raise the temperature in the quartz tube 67 to 610°C. Next, the flow control valves 63a and 63b are opened, and the dilution gas and the carrier gas are respectively supplied from the dilution gas source 62a and the carrier gas source 62b to the quartz tube 67. After the atmosphere of the quartz tube 67 is fully replaced with the dilution gas and the carrier gas, the flow rate of the dilution gas is adjusted to 0.6L/min, and the flow rate of the carrier gas is adjusted to 1.2L/min. Nitrogen gas is used as the dilution gas and the carrier gas.

(3c) Film formation

The ultrasonic oscillator 66 is vibrated at 2.4 MHz, and the vibration is transmitted to the raw material solution 64a through the water 65a, thereby atomizing the raw material solution 64a to generate a spray 64b. The spray 64b is introduced into the quartz tube 67 as a film forming chamber through the dilution gas and the carrier gas, and reacts in the quartz tube 67 to form a film on the substrate 69 through the CVD reaction on the surface of the substrate 69. In this way, a crystalline semiconductor film (semiconductor layer) is obtained. The film formation time is 60 minutes.

(4) Evaluation of semiconductor films

(4a) Surface EDS

EDS measurement was performed on the surface of the obtained film. As a result, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b)EBSD

Using a SEM (SU-5000 manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction (EBSD) device (Nordlys Nano manufactured by Oxford Instruments), inverse pole figure orientation mapping of the Ga oxide film surface was performed within a field of view of 500 μm×500 μm. The EBSD measurement conditions are as follows.

＜EBSD measurement conditions＞

Accelerating voltage: 15kV

Point Strength: 70

Working distance: 22.5mm

· Step size: 0.5μm

· Sample tilt angle: 70°

·Measurement program: Aztec (version 3.3)

The obtained inverse pole figure orientation mapping shows that the Ga oxide film has a corundum-type crystal structure with the c-axis oriented in the substrate normal direction. However, a domain deviating by 60° was confirmed in the in-plane direction. This indicates that a uniaxially oriented film containing α-Ga ₂ O ₃ was formed.

(4c) Planar TEM of the film-forming side surface

In order to evaluate the crystal defect density of the α-Ga ₂ O ₃ film, a planar TEM observation (top view) was performed. The film was cut in a manner that included the surface of the film-forming side, and the sample thickness (T) around the measurement field of view was processed by ion milling to be 150 nm. The obtained slices were subjected to TEM observation using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) at an acceleration voltage of 300 kV to evaluate the crystal defect density. In fact, TEM images with a measurement field of view of 4.1μm×3.1μm were observed in 8 fields of view. As a result, a large number of crystal defects were observed in the obtained TEM image, and it was found that the crystal defect density was at least 1.0×10 ¹¹ /cm ² or more.

Claims (15)

1. A base substrate comprising an orientation layer for growing a nitride or oxide crystal of a Group 13 element, The base substrate is characterized in that The surface of the orientation layer on the side for crystal growth is composed of a material having a corundum type crystal structure having an a-axis length and/or a c-axis length greater than that of sapphire, The alignment layer includes: a solid solution containing two or more selected from the group consisting of α-Al ₂ O ₃ , α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , α-Ti ₂ O ₃ , α-V ₂ O ₃ , and α-Rh ₂ O ₃ ; A gradient composition region exists in the alignment layer, where the composition changes in the thickness direction. 2. The base substrate according to claim 1, characterized in that The base substrate is used for crystal growth of a semiconductor layer composed of α-Ga ₂ O ₃ or an α-Ga ₂ O ₃ -based solid solution, and the orientation layer is composed of a material containing a solid solution of α-Cr ₂ O ₃ and a different material. 3. The base substrate according to claim 1 or 2, characterized in that: The a-axis length and/or c-axis length at the surface of the alignment layer is longer than the a-axis length and/or c-axis length at the back side of the alignment layer by more than 2.5%. 4. The base substrate according to claim 1 or 2, characterized in that: The material having the corundum type crystal structure includes a solid solution containing two or more selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , and α-Ti ₂ O ₃ , or a solid solution containing α-Al ₂ O ₃ and one or more selected from the group consisting of α-Cr ₂ O ₃ , α-Fe ₂ O ₃ , and α-Ti ₂ O ₃ . 5. The base substrate according to claim 1 or 2, characterized in that: The entire alignment layer is composed of a material having the corundum type crystal structure. 6. The base substrate according to claim 1 or 2, characterized in that: The a-axis length of the material having the corundum-type crystal structure at the surface is greater than And for the following. 7. The base substrate according to claim 6, characterized in that The length of the a-axis is 8. The base substrate according to claim 1 or 2, characterized in that: The alignment layer has: a compositionally stable region located close to the surface and having a stable composition in a thickness direction; and a gradient composition region located far from the surface and having a composition varying in a thickness direction. 9. The base substrate according to claim 1 or 2, characterized in that: The gradient composition region is composed of a solid solution containing α-Cr ₂ O ₃ and α-Al ₂ O ₃ . 10. The base substrate according to claim 8, characterized in that In the gradient composition region, the Al concentration decreases toward the composition stable region in the thickness direction. 11. The base substrate according to claim 1 or 2, characterized in that: The alignment layer is a heteroepitaxial growth layer. 12. The base substrate according to claim 1 or 2, characterized in that: A supporting substrate is further provided on the side of the alignment layer opposite to the surface. 13. The base substrate according to claim 12, characterized in that The supporting substrate is a sapphire substrate. 14. The base substrate according to claim 1 or 2, characterized in that: The alignment layer is a heteroepitaxial growth layer of a sapphire substrate. 15. A method for manufacturing a base substrate according to any one of claims 1 to 14, characterized in that it comprises the following steps: Preparing a sapphire substrate; forming an oriented precursor layer comprising the following material on the surface of the sapphire substrate, the material being a material having a corundum type crystal structure whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire, or a material having a corundum type crystal structure whose a-axis length and/or c-axis length are larger than the a-axis length and/or c-axis length of sapphire through heat treatment; and The sapphire substrate and the alignment precursor layer are heat-treated at a temperature above 1000°C.